1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic field sensor in the current-perpendicular-to-plane (CPP) configuration, more specifically to the use of a novel big-layer lift-off mask to pattern such a sensor having an ultra-narrow track width.
2. Description of the Related Art
Magnetic read sensors that utilize the giant magnetoresistive (GMR) effect for their operation must be patterned to produce a required trackwidth. Such patterning is conventionally done using a single photolithographic lift-off mask as both an etching stencil and a deposition mask. The shape of the stencil portion of such a mask permits the necessary trimming of the deposited layers to the required trackwidth and then the mask is used to allow deposition of additional layers (eg. conduction lead layers, biasing layers and/or insulation layers) within the removed regions. If the trackwidth of the read element is to be held below about 0.1 microns, then the prior art methods of forming the prior art masks have notable shortcomings. Han et al. (U.S. patent application Ser. No. 6,493,926), assigned to the same assignee as the present invention and which is fully incorporated herein by reference, discusses several problems associated with prior art lift-off masks in which an upper (stencil) layer of photoresist is formed over a lower, undercut, pedestal, layer. In such mask designs the width of the pedestal layer becomes a critical factor in the proper performance of the mask during the deposition stage. If the pedestal is undercut too much, the upper portion of the mask can collapse prematurely under the weight of deposition residue making a clean lift-off of the mask impossible. On the other hand, if the pedestal is insufficiently undercut, subsequent depositions can build up against the pedestal, called “fencing,” leading to excessive thicknesses of the deposited material and short-circuiting of conductive layers. To overcome the difficulties of forming properly and consistently undercut pedestals and for use in forming trackwidths of approximately 0.5 microns, Han et al. teach the formation of a big-layer suspension-bridge mask formation, in which there is no pedestal directly beneath the upper portion of the mask, but wherein the upper portion is supported on two pedestals that are laterally disposed beneath two distal ends of the mask. The complete elimination of any support directly beneath the mask thereby avoids the problems associated with insufficient or overly-sufficient pedestal undercut. The formation taught by Han et al. requires that the portion of the mask that would ordinarily be beneath the upper portion be completely removed, so that the upper portion is suspended above the device to be patterned and does not contact it. This object is achieved by forming the pedestal portion of the mask of a layer of PMGI, while forming the upper portion of the mask of a layer of photoresist material. Application of a proper developing solution thereupon dissolves the lower PMGI portion preferentially relative to the photoresist upper portion, removing the PMGI except beneath the end portions where it remains to serve as a support.
Fontana J R., et al. (US Patent Application Publication No. US 2002/0167764 A1) also teach the formation of a suspension bridge type big-layer lift-off mask in which a layer of PMGI (polydimethylglutarimide) polymer is first spun onto a substrate and then a layer of PMMA is spun over the PMGI layer. An e-beam is then used to form a mask pattern in the upper layer by developing the upper layer and the PMGI layer is dissolved to form the undercut region.
The method taught by Han et al. was applied to patterning trackwidths on the order of 0.5 microns. Attempts to apply the method of Han et al. to produce trackwidths below 0.1 microns discloses insufficiencies in that mask design. In particular, the suspended photoresist portion of the mask must be narrowed to such a degree relative to its length that it sags and contacts the substrate directly beneath it. An additional problem occurs when the void portion beneath the suspended portion is so large that subsequent depositions cover portions of the substrate beneath the bridge (“overspray”), leading to inconsistent definition of the trackwidth.
Referring to FIG. 1a there is shown, schematically, an overhead view of a suspension-bridge mask of the type taught by Han et al., formed in a “dog-bone” shape, wherein a narrow central portion (5) of the upper photoresist portion of the mask are supported by its distal ends (10), which are flared outward and rest on lower, undercut pedestal regions (20), which cannot be seen from above and are shown in broken line outline. FIG. 1b shows a cross-sectional transverse view of the same mask (taken through the center line (7) of FIG. 1a), shown above a substrate (50) indicating that the length and width of the central portion (5) are in a proper relationship relative it its thickness so that it remains properly suspended (9) above the substrate.
Referring next to FIG. 2a, there is shown a side view of a mask similar to that in FIGS. 1a,b, except that the suspended central region (5) is narrower than that in FIGS. 1a,b, causing it to sag between its supports (20) and contact the substrate (50). Referring next to FIG. 2b, there is shown a cross-sectional view of a mask similar to that in FIG. 2a taken transversely through the narrow portion. In this mask, the central suspended portion (5) does not sag, but rather leaves to great a space between itself and the substrate (50). The deposition of a layer (30), such as a dielectric layer, a conducting lead layer or a magnetic biasing layer, produces undesirable regions of deposition (40), called overspray, beneath the suspended region. These undesirable regions effectively reduce the trackwidth region in an uncontrollable manner.
In order to retain the advantageous properties of a suspension bridge type mask as are set forth in detail in Han et al., yet to eliminate problems such as sagging or excessive space beneath the suspended portion as the mask is formed for use in increasingly narrow patterning processes, the present invention teaches a novel, modified, suspension bridge type lift-off mask in which the central suspended portion is rendered incompletely suspended by the formation of a thin ridge that runs between the distally located bridge supports formed from the underlayer, which ridge maintains the bridge at a fixed height relative to the substrate. The formation of such a ridge requires that the underlayer be very carefully etched so that the ridge offers mechanical stability and is reproducible, yet still eliminates the problems of fencing and mask collapse.
Prior art methods of dissolving the lower PMGI layer (eg. use of organic solvents or anisotropic plasma etches) to form the suspended bridge are either insufficiently controllable or damaging to the upper layer to be used to form the thin ridge structure. Therefore, the present method introduces a novel ozone oxidation method which effectively retards the dissolution rate of the PMGI in an organic solvent, rendering the rate and degree of undercut controllable with a high degree of precision. The use of ozone in an etching ambient is known in the prior art, where it has been applied to the etching of certain layers having an alloyed or elemental metal composition. Morgan et al. (US Patent Application Publication No.: US 2003/0170961 A1) teaches the etching of a portion (millimeters in dimension) of a layer formed of metals such as platinum, ruthenium, rhodium, palladium, iridium and their mixtures in an ambient comprising a halogenide, ozone and H2O. The method taught therein, however, does not contemplate the controlled etching of a PMGI layer to form an etch mask which is dimensionally less than a micron in width.